This invention relates to apparatus for simultaneously reading multiple tracks of an optical storage medium. More particularly, the invention relates to providing an optical pickup that produces multiple reading beams from a single source, and which has optical characteristics superior to those of previous similar systems.
Due to their high storage density, long data retention life, and relatively low cost, optical disks have become popular as a means to distribute information. Large format disks have been developed for storing full length motion pictures. The compact disk (CD), and more recent mini disk (MD) formats were developed and marketed for the distribution of musical recordings and have essentially replaced vinyl records. High-capacity, read-only data storage media, such as CD-ROM, have become prevalent in the personal computer field, while the new Digital Versatile Disk (DVD) format may soon replace videotape as the distribution medium for video information.
An optical disk is made of a transparent disk or substrate in which data, in the form of a serial bit-stream, is encoded as a series of pits in a reflective surface within the disk. Data is read from the optical disk by focusing a low power laser beam onto a track on the disk and detecting the light reflected from the surface of the disk. By rotating the optical disk, the light reflected from the surface of the disk is modulated by the pattern of the pits rotating into and out of the laser""s field of illumination. Optical systems detect the modulated, reflected, laser light and produce an electrical signal which may be decoded to recover the digital data stored on the optical disk. The recovered digital data, which may include error correcting codes and additional subcoded information, is further processed to recover the stored data.
To be able to retrieve data from anywhere on an optical disk, an optical system includes a pickup assembly which may be positioned to read data from any disk track. An example of an integrated optical pickup assembly, wherein the laser source of illumination, focusing optics, and the detector that receives reflected illumination from the optical disk are contained within a single compact pickup assembly is described in U.S. Pat. No. 5,285,062. Servo mechanisms are provided for focusing the optical system and for keeping the pickup assembly positioned over the track, despite disk warpage or eccentricity.
In most previously known systems the data is retrieved from the disk serially, i.e. one bit at a time, so that the maximum data transfer rate for the optical disk reader is determined by the rate at which the pits pass by the pickup assembly. The linear density of the bits and the track pitch are fixed by the specification of the particular optical disk format. For example, CD disks employ a track pitch of 1.6 xcexcm, while the DVD employs a track pitch only about one-half as wide.
Commercialized methods of increasing the data transfer rate of optical disk readers have focused on increasing the rate at which the pits pass by the pickup assembly by increasing the rotational speed of the disk itself. Currently, drives with rotational speeds of 2xc3x97 to 12xc3x97standard speed are commercially available. However, higher disk rotational speeds place increased demands on the optical and mechanical subsystems within the optical disk player, making such players more difficult and expensive to design and manufacture.
Another previously known technique for increasing the average data transfer rate involves simultaneously reading multiple tracks of data. For example, U.S. Pat. No. 5,144,616 to Yasukawa et al. describes a system for generating multiple reading beams using multiple laser sources which are spaced apart by predetermined distances. U.S. Pat. No. 4,459,690 to Corsover et al. describes a multi-beam reading system in which an illumination beam generated by a single laser source is split into multiple beams using an acousto-optic device that dithers the beam in a direction normal to the track direction.
A drawback common to the foregoing multi-beam systems is that the generation of multiple beams requires either multiple laser sources or the use of complicated acousto-optic devices. In either event, both the manufacturing complexity and overall size of the pickup system increases, leading to difficulties in aligning the optics and the detectors, increased power requirements, and increased overall cost.
Copending U.S. patent application Ser. No. 08/804,105 describes tracking and data processing circuitry for a system to increase disk reading speeds by using multiple beams which are generated using a diffractive element to read multiple tracks simultaneously. In the system described, as well as other multi-beam systems which use diffractive elements to split a single beam into multiple beams, the diffractive element is typically placed in the optical path immediately after the laser diode. The small distance between the laser and the diffractive element requires use of a diffractive element having a relatively small period.
The small period of the gratings used in such multi-beam systems makes the gratings difficult and expensive to manufacture. Off-axis aberrations also increase the field curvature of the optical system, so that the best focus locations of the outermost beams are different than the best focus location for the central beam. Thus, when the central beam is in focus, the outermost beams may be out of focus.
In view of the foregoing, it would be desirable to provide a multi-beam optical pickup for simultaneously reading multiple tracks of an optical storage medium that uses a diffractive element to generate the beams, and that has improved optical characteristics, such as reduced vignetting and reduced aberration of the outermost beams.
It also would be desirable to provide a multi-beam pickup having fewer optical components, and components which are easier to manufacture, reducing the cost of producing the optical pickup.
In view of the foregoing, it is an object of the present invention to provide a multi-beam optical pickup for simultaneously reading multiple tracks of an optical storage medium that uses a diffractive element to generate multiple beams, and that has improved optical characteristics compared to previously known systems.
It is a further object of the invention to provide a multi-beam pickup assembly that contains fewer parts than previously known systems, and is easy to manufacture, thereby reducing the cost of the system.
These and other objectives of the invention are accomplished by providing a multi-beam optical pickup wherein the diffractive element, used to generate the beams, is placed after a collimator in the optical path. This arrangement provides a greater distance from the laser source to the diffractive element than is found in previously known systems. This greater distance permits a diffractive element having a relatively large period and a small angle between diffractive orders to be used, thereby reducing vignetting and optical aberrations in the outermost beams. The new placement of the grating, after the collimating lens, reduces the off-axis aberrations of the outer beams. Reduction of off-axis aberrations also decreases the field curvature of the optics, and enables more beams to be used in the multi-beam system.
The larger period of the diffractive element also makes the diffractive element easier and less costly to manufacture. By combining diffractive and holographic elements used in the optical pickup with other optical elements, the number of parts in the optical pickup may be reduced, further reducing the complexity and cost of manufacturing.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.